This invention relates to a method of providing a leveling property to photocurable resin composition layers in a production process of three-dimensional objects by photohardening.
Japanese Patent Publication Tokkai 62-101408 and European Patent Publication EPA 250121 disclosed a method of forming a layer of photocurable resin composition containing a photocurable liquid and a photoinitiator, photohardening at least a portion of this layer by exposing it to actinic radiation such as an ultraviolet laser beam, thereafter repeating the process of forming thereabove a new layer of photocurable resin composition and photohardening at least a portion of this layer by exposing it to actinic radiation, and thereby forming a three-dimensional photohardened single object from continuous layers of photocurable resin composition. As disclosed in Japanese Patent Publications Tokko 6-61848 and 6-61849, photocurable resin compositions containing different kinds of fillers are also used in such a production process of three-dimensional objects for limiting the depth of invasion by the actinic radiation or for improving the physical properties of the obtained three-dimensional photohardened objects.
In such a production process of a three-dimensional photohardened object, it is important to make the surface of the photocurable resin composition layer sufficiently smooth (or to level it) before it is exposed to actinic radiation. It is because, if the layer surface is not sufficiently leveled and hence is uneven, the degree and extent of photohardening cannot be constant, depending on the position of the exposed photocurable resin composition, and this affects significantly the physical properties and shape precision of the three-dimensional object thus formed. Such a leveling process is particularly important when photocurable resin compositions containing fillers are used. The presence of a filler alone affects the leveling property of a photocurable resin composition adversely. In certain cases, the photocurable resin composition may totally lose its leveling or smoothing property.
In the past, mechanical means such as a doctor blade have been used to level the surface of a photocurable resin composition. In the case of a photocurable resin composition containing a filler, however, its layer cannot be smoothed or leveled by such mechanical means alone. It is therefore a common practice at the present time to wait for a long time for natural leveling to take place after a processing by a mechanical means. It now goes without saying that it affects the workability and productivity adversely if one has to wait for such a long time. If an actinic radiation beam is applied after waiting only a short time before the photocurable resin composition layer becomes sufficiently leveled, however, the physical properties and shape precision of the three-dimensional object thus formed become poor, as explained above. In particular, photocurable resin compositions without any leveling property cannot be used for the production of a three-dimensional object at all.
For leveling a photocurable resin composition layer containing a filler, a method of adding a photocurable monomer with a low molecular weight as a constituent photocurable liquid of the photocurable resin composition so as to reduce the overall viscosity, a method of reducing the relative content of the filler and methods of selecting the kinds of filler may be considered, but these methods alone cannot make it possible to obtain three-dimensional objects with required physical properties and shape precision, and the leveling property can be provided only to a limited extent.